Multiple quantum well broad spectrum gain medium and method for forming same

ABSTRACT

A broadband medium ( 100 ) for a laser ( 300 ) having multiple quantum wells ( 130 ).

This application claims priority to U.S. Provisional Application No.60/269,267, filed Feb. 20, 2001, which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor optical gain media and, moreparticularly, to a multiple quantum well semiconductor optical gainmedium that exhibits a broad gain spectrum.

2. Background of the Related Art

Broadly tunable optical devices, such as broadly tunable semiconductorlasers and broadband wavelength converters, are desired for variousoptical communication applications, such as optical networking,wavelength-division-multiplexing and other telecommunicationsapplications.

Multiple quantum well (MQW) gain materials have been used because oftheir relatively broad gain spectra. However, there is a continuing needfor broader tuning ranges than prior art MQW materials offer.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE INVENTION

An object of the invention is to solve at least the above problemsand/or disadvantages and to provide at least the advantages describedhereinafter.

The present invention provides a broadband gain medium, and a method forforming the same, that exhibits a broader gain spectrum than prior artsemiconductor materials. The broadband gain medium of the presentinvention includes a multiple quantum well region made up of at leasttwo quantum wells, with at least one of the quantum wells having athickness and composition that vary as a function of position along theresonant cavity direction, and at least one quantum well having athickness profile that is different than the other quantum wells.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIGS. 1A-1C are cross-sectional, perspective and plan schematic views,respectively, of a broadband gain medium, in accordance with oneembodiment of the present invention;

FIGS. 2A-2C are schematic representations of the conduction and valencebands of a preferred multiple quantum well region used in the broadbandgain medium of FIGS. 1A-1C, at three different points along thewaveguide direction, in accordance with the present invention;

FIG. 2D is a schematic representation of the conduction and valencebands of one of the quantum wells in the Broadband gain medium of FIGS.1A-1C, at three points along the waveguide direction, in accordance withthe present invention; and

FIG. 3 is a schematic representation of a tunable laser utilizing theBroadband gain medium of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1A-1C are cross-sectional, perspective and plan schematic views,respectfully, of a broadband gain medium 100,in accordance with oneembodiment of the present invention. The broadband gain medium 100includes a substrate 110, a buffer layer 120, a MQW region 130, acladding layer 140 and a contact layer 150.

In a preferred embodiment, the substrate 110 is an n-doped InPsubstrate, the buffer 120 is a n-doped InP buffer layer, the MQW region130 is an InGaAs/InGaAsP MQW, the cladding layer 140 is a P-doped InPlayer, and the contact layer is a P-doped InGaAs layer. However, othermaterials can be used for the substrate 110, the buffer layer 120, theMQW 130, the cladding layer 140 and the contact layer 150, while stillfalling within the scope of the present invention.

The MQW region 130 preferably comprises four quantum wells 130 a-130 d,preferably InGaAs quantum wells, separated by five barriers 135 a-135 e,preferably InGaAsP barriers, that are preferably formed using standardmetalorganic chemical vapor deposition (MOCVD) techniques. At least oneof the quantum wells 130 a-130 d has a non-constant “thickness profile”and a non-constant material composition that each vary as a function ofposition along the x-axis. The term “thickness profile”, as used herein,refers to the thickness of the quantum well (measured along the z-axis)at all positions along the resonant cavity direction (x-axis). Thus, aquantum well with a constant thickness profile has a substantiallyconstant thickness along its entire length (along the x-axis), while aquantum well with a non-constant thickness profile has a thickness thatchanges as a function of position along the x-axis.

In a preferred embodiment, each of the quantum wells 130 a-130 d has anon-constant thickness profile, preferably a thickness that starts at aninitial value at one end 160 of the MQW region 130, and that increasesas a function of position along the x-axis, as well as a compositionthat varies as a function of position along the x-axis. Pot ease ofillustration, the thickness increase along the x-axis is graphicallyshown only in FIGS. 2A-2D, which will be explained in more detail below.

Further, at least one of the quantum wells 130 a-130 d preferably has adifferent thickness profile than the other quantum wells. In a preferredembodiment, all the quantum wells 130 a-130 d each have differentthickness profiles with respect to each other. Thus, each quantum wellpreferably has different thickness with respect to the other quantumwells at all points along the x-axis.

A non-constant thickness profile is preferably achieved by usingselected area growth (SAG) techniques. With SAG, growth inhibition froma mask, preferably an SiO₂ mask, is used to enhance the growth rate inbetween the mask regions. During MOCVD, no deposition takes place on themask, therefore growth rate enhancement occurs in the unmasked regions.

As shown in FIGS. 1B and 1C, a preferred mask for selected area growthof each of the quantum wells 130 a-130 d comprises a symmetric pair oftapered SiO₂ stripes 200 a and 200 b that are deposited and patterned onthe substrate 110 using standard photolithographic techniques.

For a given separation between each of the SiO₂ stripes 200 a and 200 b,the width of the SiO₂ stripes determines the growth rate enhancement inthe region between the stripes 200 a and 200 b. Because each SiO₂ stripe200A and 200B is tapered, the quantum wells 130 a-130 d and barriers 135a-135 e grown between the stripes 200A and 200B will exhibit a variationin thickness along their length for any predetermined growth time. Thisproduces the non-constant thickness profile.

In addition, the width of the SiO₂ oxide stripes 200 a and 200 bdetermines the material composition of the quantum well layers andbarrier layers grown between the stripes. In the case of anIn_(1-x)Ga_(x)As_(1-y)P_(y) barrier layer, the barrier layer becomesmore Indium and Arsenide rich (smaller x and y) as the width of the SiO₂oxide stripes 200 a and 200 b increases. In the case of anIn_(1-x)Ga_(x)As quantum well, the quantum well becomes more Indium rich(smaller x) as the width of the SiO₂ oxide stripes 200 a and 200 bincreases.

The SiO₂ stripes 200 a and 200 b are shown in FIGS. 1B and 1C forillustrative purposes. It should be appreciated that the oxide stripesare removed after fabrication of the Broadband gain medium 100.

As discussed above, in addition to each quantum well having anon-constant thickness profile and a non-constant material composition,the growth time used for at least one of the quantum wells 130 a-130 dis different than the growth time used for the other quantum wells, sothat at least one of the quantum wells has a different thickness profilethan the other quantum wells. A quantum well with a different thicknessprofile than the other quantum wells will exhibit a different thicknessthan the other quantum wells at all points along the x-axis. In apreferred embodiment, a different growth time is used for each of thequantum wells 130 a-130 d, so that each quantum well exhibits a uniquethickness profile.

The variation in thickness and material composition along the x-axisexhibited by each of the quantum wells, which is preferably obtained byusing SAG growth techniques, results in a varying band gap as a functionof position along the x-axis for each of the quantum wells 130A-130D.This, in turn, varies the wavelength of peak gain as a function ofposition along the x-axis, thereby producing a broader gain spectrum forthe Broadband gain medium 100.

In addition to the broader gain spectrum caused by the non-constantthickness profile and non-constant material composition exhibited byeach quantum well, varying the growth time of each quantum well so as tochange the thickness profile of each quantum well also contributes tobroadening of the optical gain spectrum of the broadband gain medium100. This is because changing the thickness profiles will also vary theband gap, thus broadening the overall gain spectrum of the broadbandgain medium 100.

The variation in the band gap of the quantum wells 130 a-130 d caused byeach quantum well having a non-constant thickness profile andnon-constant material composition, as well as each quantum well having adifferent thickness profile, is shown schematically in FIGS. 2A-2D.FIGS. 2A-2C are schematic representations of the conduction and valencebands of each quantum well, as a function of position along the z-axis,at positions X₀, X₁, and X₂ along the x-axis, respectively. FIG. 2D is aschematic representation of the conduction and valence bands of quantumwell 130 c, as a function of position along the z-axis, at x-axispositions X₀, X₁, and X₂.

As shown in FIG. 2A, at position X₀ along the x-axis (see FIG. 1B), eachof the quantum wells 130 a-130 d exhibit a different thickness, with thequantum well 130 a closest to the buffer layer 120 exhibiting thesmallest thickness, and the other quantum wells 130 b-130 d exhibitingprogressively larger thicknesses as they get closer to the claddinglayer 140. This is the result of each quantum well having a differentthickness profile.

The different quantum well thicknesses result in different valence bandenergies, with the narrowest quantum well 130 a exhibiting the highestvalence band energy 138 a and the thickest quantum well 130 d exhibitingthe lowest valence band energy 138 d. This results in each of thequantum wells 130 a-130 d providing a peak gain for a differentrespective wavelength band, which is determined by the respectivevalence band energy levels 138 a-138 d. The smaller the valence bandenergy, the longer the wavelength at which peak gain is provided.

As illustrated in FIGS. 2B and 2C, the thickness of each of the quantumwells 130 a-130 d gets progressively larger as one moves along thex-axis. This is because each of the quantum wells 130 a-130 d arepreferably formed with a non-constant thickness profile, in which thethickness progressively increases along the x-axis. In addition, thematerial composition of each quantum well varies as a function ofposition along the x-axis, as discussed above.

This is also illustrated in FIG. 2D, which shows the conduction andvalence bands for a single quantum well 130 c at points X₀, X₁, and X₂along the x-axis. As shown in FIG. 2D, each individual quantum well hasa thickness that preferably increases along the x-axis, as well as amaterial composition that varies as a function of position along thex-axis. Thus, the valance band energy 138 c of the quantum well 130 cgets progressively lower along the x-axis.

In a preferred embodiment, quantum well 130 a has a thickness thatvaries from approximately 2.4 nm to approximately 6.0 nm as a functionof position along the x-axis, quantum well 130 b has a thickness thatvaries from approximately 2.8 nm to approximately 7.0 nm as a functionof position along the x-axis, quantum well 130 c has a thickness thatvaries from approximately 3.2 nm to approximately 8.0 nm as a functionof position along the x-axis, and quantum well 130 d has a thicknessthat varies from approximately 3.6 nm to approximately 9.0 nm as afunction of position along the x-axis.

The broadband gain medium 100 of the present invention can be used tomake a highly tunable laser, as shown in FIG. 3. The tunable laser 300is an external cavity laser that includes the broadband gain medium 100,a grating 310 and lenses 320 and 330. Wavelength tuning of the laseroutput 340 is achieved by rotating the grating 310. The grating is oneexample of a wavelength tuning device that can be used to tune thewavelength of the tunable laser 300. It should be appreciated that otherwavelength tuning devices, such as a Fabry-Perot filter, may be usedwhile still falling within the scope of the present invention.

With the broad gain spectrum exhibited by the broadband gain medium 100of the present invention, the tunable laser 300 can exhibit a very broadtuning range. With proper adjustment of the thickness of each quantumwell along the x-axis, as well as the thickness profile of each quantumwell, a tuning range as large as approximately 500 nm or more can beachieved.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art.

For example, although an InGaAs/InGaAsP multiple quantum well region hasbeen described and illustrated as one embodiment, other types ofmultiple quantum well regions, such as InAlGaAs/InGaAs and AlGaSb/GaSbmultiple quantum well regions, can be used while still falling withinthe scope of the present invention.

Further, the embodiment described and illustrated above includes fourquantum wells, with each quantum well having a non-constant thicknessprofile. In addition, each of the quantum wells in the above-describedembodiment exhibits a different thickness profile. It should beappreciated that any multiple quantum well region can be used, as longas at least one of the quantum wells has a non-constant thicknessprofile, and at least one of the quantum wells has a thickness profilethat is different than the other quantum wells. For example, the presentinvention can be practiced in whole or in part by a multiple quantumwell region that includes three quantum wells, with only one of thequantum wells having a non-constant thickness profile, and one of thequantum wells having a thickness profile that is different than thethickness profile of the other two quantum wells.

In addition, in the embodiment described and shown above, thenon-constant thickness profile is obtained by using SAG fabricationtechniques, and the thickness profile of each of the quantum wells ismade different from the others by varying the growth time of eachquantum well layer. However, other techniques known in the art forachieving these thickness parameters may be used while still fallingwithin the scope of the present invention. In the claims,means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures.

1. A broadband gain medium, comprising: a substrate; and a multiplequantum well region on the substrate comprising at least two quantumwells, wherein at least one of the quantum wells exhibits a non-constantthickness profile and a non-constant material composition, and at leastone of the quantum wells exhibits a thickness profile that is differentthan a thickness profile of the other quantum wells.
 2. The broadbandgain medium of claim 1, wherein the multiple quantum well regioncomprises an InGaAs/InGaAsP quantum well region.
 3. The broadband gainmedium of claim 1, wherein the multiple quantum well region comprises aplurality of InGaAs quantum wells, wherein respective InGaAsP layers arepositioned between adjacent InGaAs quantum wells.
 4. The broadband gainmedium of claim 1, wherein the thickness profile of each quantum well isadjusted to broaden a gain spectrum of the broadband gain medium.
 5. Thebroadband gain medium of claim 1, wherein each of the quantum wells hasa non-constant thickness profile and a non-constant materialcomposition.
 6. The broadband gain medium of claim 1, wherein eachquantum well has a different thickness profile.
 7. The broadband gainmedium of claim 1, wherein each quantum well has a different andnon-constant thickness profile and a non-constant material composition.8. The broadband gain medium of claim 5, wherein each quantum well has athickness that increases along a resonant cavity direction.
 9. Thebroadband gain medium of claim 7, wherein each quantum well has athickness that increases along a resonant cavity direction.
 10. Atunable semiconductor laser comprising the broadband gain medium ofclaim
 1. 11. A broadband gain medium, comprising: a substrate; a bufferlayer on the substrate; a multiple quantum well region on the bufferlayer comprising at least two quantum wells, wherein at least one of thequantum wells exhibits a non-constant thickness profile and anon-constant material composition, and at least one of the quantum wellsexhibits a thickness profile that is different than a thickness profileof the other quantum wells; a cladding layer on the multiple quantumwell region; and a contact layer on the cladding layer.
 12. Thebroadband gain medium of claim 11, wherein the substrate comprises ann-doped InP substrate, the buffer layer comprises an n-doped InP bufferlayer, the multiple quantum well region comprises an InGaAs/InGaAsPmultiple quantum well region, the cladding layer comprises a p-doped InPlayer, and the contact layer comprises a p-doped InGaAs layer.
 13. Thebroadband gain medium of claim 11, wherein the multiple quantum wellregion comprises a plurality of InGaAs quantum wells, and respectiveInGaAsP layers positioned between adjacent InGaAs quantum wells.
 14. Thebroadband gain medium of claim 11, wherein the multiple quantum wellregion comprises: a first InGaAsP layer on the buffet layer; a firstInGaAs quantum well on the first InGaAsP layer; a second InGaAsP layeron the first InGaAs quantum well; a second InGaAs quantum well on thesecond InGaAsP layer; a third InGaAsP layer on the second InGaAs quantumwell; a third InGaAs quantum well on the third InGaAsP layer; a fourthInGaAsP layer on the third InGaAs quantum well; a fourth InGaAs quantumwell on the fourth InGaAsP layer; and a fifth InGaAsP layer on thefourth InGaAs quantum well.
 15. The broadband gain medium of claim 11,wherein each of the quantum wells has a non-constant thickness profileand a non-constant material composition.
 16. The broadband gain mediumof claim 11, wherein each of the quantum wells has a different thicknessprofile.
 17. The broadband gain medium of claim 11, wherein each of thequantum wells has a different and non-constant thickness profile, and anon-constant material composition.
 18. The broadband gain medium ofclaim 14, wherein each of the quantum wells has a thickness thatincreases along a resonant cavity direction.
 19. The broadband gainmedium of claim 18, wherein, for any point along the resonant cavitydirection, the thickness of the second InGaAs quantum well is largerthan the thickness of the first InGaAs quantum well, the thickness ofthe third InGaAs quantum well is larger than the thickness of the secondInGaAs quantum well, and the thickness of the fourth InGaAs quantum wellis larger than the thickness of the third InGaAs quantum well.
 20. Atunable semiconductor laser comprising the broadband gain medium ofclaim
 19. 21. A tunable semiconductor laser, comprising: a broadbandgain medium, comprising: a substrate, a buffer layer on the substrate, amultiple quantum well region on the buffer layer comprising at least twoquantum wells, wherein at least one of the quantum wells exhibits anon-constant thickness profile and a non-constant material composition,and at least one of the quantum wells exhibits a thickness profile thatis different than a thickness profile of the other quantum wells, acladding layer on the multiple quantum well region, and a contact layeron the cladding layer; and a wavelength tuning device optically coupledto the broadband gain medium.
 22. The tunable laser of claim 21, whereinthe wavelength tuning device comprises a grating.
 23. A method offabricating a broadband gain medium, comprising the steps of: growing atleast two quantum wells by metalorganic chemical vapor deposition(MOCVD) selective area growth such that at least one of the quantumwells has a non-constant thickness profile and a non-constant materialcomposition; wherein a growth time for each of the quantum wells isadjusted such that a thickness profile of at least one of the quantumwells is different than a thickness profile of the other quantum wells.24. The method of claim 23, wherein the step of growing at least twoquantum wells by MOCVD selective area growth comprises the steps of:forming an oxide mask on a substrate, wherein the oxide mask comprisesfirst and second tapered oxide mask regions spaced apart on thesubstrate; and subjecting the substrate to MOCVD, wherein a quantum wellgrowth rate between the first and second tapered oxide regions varies asa function of the width of the first and second tapered oxide regions.25. The method of claim 23, wherein at least one of the quantum wells isformed so that its thickness increases along a resonant cavitydirection.
 26. The method of claim 23, wherein four quantum wells areformed with respective thicknesses that increase along a resonant cavitydirection.
 27. A method of fabricating a semiconductor opticalamplifier, comprising the steps of: providing a substrate; forming abuffer layer on the substrate; growing a multiple quantum well region,comprising a plurality of quantum wells, on the buffet layer usingmetalorganic chemical vapor deposition (MOCVD) selective area growth,such that at least one of the quantum wells has a non-constant thicknessprofile and a non-constant material composition, wherein a growth timefor each of the quantum wells is adjusted such that a thickness profileof at least one of the quantum wells is different than a thicknessprofile of the other quantum wells; and forming a cladding layer on themultiple quantum well region.
 28. The method of claim 27, wherein thestep of forming a multiple quantum well region by MOCVD selective areagrowth comprises the steps of: forming an oxide mask on the substrate,wherein the oxide mask comprises first and second tapered oxide maskregions spaced apart on the substrate; and subjecting the substrate toMOCVD, wherein a quantum well growth rate between the first and secondtapered oxide regions varies as a function of the width of the first andsecond tapered oxide regions.
 29. The method of claim 27, wherein atleast one of the quantum wells is selectively grown so that itsthickness increases along a resonant cavity direction.
 30. The method ofclaim 27, wherein four quantum wells are selectively grown withrespective thicknesses that increase along a resonant cavity direction.